This invention relates to radio broadcasting, and more particularly, to AM In-Band-On-Channel (IBOC) Digital Audio Broadcasting (DAB), and signal processing in AM IBOC DAB transmitters.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. AM IBOC DAB can be transmitted in a hybrid format where the digital signal coexists with the analog modulated signal, or it can be transmitted in an all-digital format where the removal of the analog signal enables improved digital coverage with reduced interference. IBOC DAB requires no new spectral allocations because each DAB signal is simultaneously transmitted within the spectral mask of an existing AM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
U.S. Pat. No. 5,588,022 discloses a hybrid AM IBOC broadcasting method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel that includes the steps of broadcasting an amplitude modulated radio frequency signal having a first frequency spectrum, wherein the amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal, and simultaneously broadcasting a plurality of digitally modulated carrier signals within a bandwidth which encompasses the first frequency spectrum, each of the digitally modulated carrier signals being modulated by a portion of a digital program signal. A first group of the digitally modulated carrier signals lie within the first frequency spectrum and are modulated in-quadrature with the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal.
In the United States, the emissions of AM broadcasting stations are restricted in accordance with Federal Communications Commission (FCC) regulations to lie within a signal level mask defined such that: emissions 10.2 kHz to 20 kHz removed from the analog carrier must be attenuated at least 25 dB below the unmodulated analog carrier level, emissions 20 kHz to 30 kHz removed from the analog carrier must be attenuated at least 35 dB below the unmodulated analog carrier level, and emissions 30 kHz to 60 kHz removed from the analog carrier must be attenuated at least [35 dB+1 dB/kHz] below the unmodulated analog carrier level.
U.S. Pat. No. 5,859,876 is directed to reduction of analog AM signal envelope distortion caused by multiple digitally modulated carriers in the same channel as the analog AM signal. The signal transmission system of U.S. Pat. No. 5,859,876 includes means for transmitting an amplitude modulated carrier and a plurality of digitally modulated carriers. The digital carriers are placed both above and below the frequency of the analog AM carrier. Certain digital carriers that are above the frequency of the analog AM carrier have an associated digital carrier that is at an equal frequency offset below the analog AM carrier. The data modulation placed on the upper digital carrier and its counterpart are such that the signal resulting from their addition has no component that is in-phase with the analog AM carrier. Digital carrier pairs arranged in this way are said to be complementary. This configuration delivers fidelity improvements to analog AM reception of digital broadcast signals. U.S. Pat. No. 5,859,876 mentions that an additional means of reducing envelope distortion is to predistort the signal envelope. The signal envelope is predistorted to counteract the distortion added by the digital carriers. U.S. Pat. No. 5,859,876 discloses a predistortion operation performed using analog processing, and mentions that the operation can also be performed using digital processing.
In hybrid IBOC DAB systems, the presence of the analog modulated signal also gives rise to interference with respect to the digitally modulated signals in the output of the receiver demodulator. The orthogonal frequency division multiplexed (OFDM) waveform used to transmit digital information in the hybrid system permits easy removal of the analog modulated signal effects for the complimentary sub-carriers, the sub-carriers directly beneath the AM spectrum. The construction of these sub-carriers guarantees their orthogonality with the analog modulated signal after they are demodulated and appropriately combined. The effects of the analog modulated signal on the other, non-complimentary sub-carriers, however, cannot be removed by processing in the receiver.
This invention seeks to provide a method and apparatus for reducing distortion in the digital signal transmitted by the non-complementary carriers resulting from the analog modulated carrier of an AM IBOC hybrid digital broadcasting system.
This invention provides a method for pre-compensating an analog signal in a composite digital audio broadcasting signal including the analog signal and a plurality of digitally modulated sub-carrier signals, the method comprising the steps of sampling the analog signal to obtain successive blocks of samples, demodulating each of the blocks of samples to obtain a plurality of demodulator outputs, remodulating the demodulator outputs that correspond to the predetermined ones of the digitally modulated sub-carrier signals to produce an error signal, subtracting the error signal from the one of the blocks of samples to produce a pre-compensated block of samples, and combining the pre-compensated block of samples with a plurality of OFDM pulses to produce a compensated composite signal.
Successive blocks of samples can overlap each other. The number of samples in the pre-compensated block is preferably equal to the number of samples in an OFDM symbol. To produce the demodulator outputs that correspond to non-complementary sub-carriers in the composite signal, the demodulator outputs that correspond to complementary sub-carriers can be set to zero.
The invention also includes an apparatus for pre-compensating an analog signal in a composite digital audio broadcasting signal including the analog signal and a plurality of digitally modulated sub-carrier signals, the apparatus comprising means for sampling the analog signal to obtain successive blocks of samples, means for demodulating each of the blocks of samples to obtain a plurality of demodulator outputs, means for remodulating the demodulator outputs that correspond to the predetermined ones of the digitally modulated sub-carrier signals to produce an error signal, means for subtracting the error signal from the one of the blocks of samples to produce a pre-compensated block of samples, and means for combining the pre-compensated block of samples with a plurality of OFDM pulses to produce a compensated composite signal.